1. Field of the Invention
The invention pertains to the field of information transmission, and more particularly, to the transmission of information, without distortion, over a frequency band centered on a skirt of the transmision system passband.
2. Description of the Prior Art
Since the inception of color TV, the industry has continually strived to improve the picture quality. Though many significant advances have been realized, there is an unending clamor for still more. Presently, the demand exists for increasing the screen size from an aspect ratio of 4:3 to an aspect ratio of 51/3:3, thereby providing an additional 33% of information area. To accomplish this, and maintain compatibility with existing equipment, the additional information required for the increase aspect ratio screen must be transmitted and received in a manner that prevents picture distortion due to crosstalk between the additional information signals and the standard information signals. Complicating the problem is the FCC prescribed 6 MHz bandwidth per channel. This bandwidth restriction requires the additional information to share a frequency allocation with the standard information.
One method utilized for providing the required isolation without exceeding the 6 MHz requirement applies the additional information to a doubly-balanced modulator, which utilizes the same carrier frequency utilized for transmitting the standard information signals. A phase shift of 90.degree. is inserted in the additional information carrier relative to the phase of the standard information carrier. In this manner, two quadrature related signals, with the additional signal being of the double sidebandsuppressed carrier type, are provided at the output terminals of the modulators which may be simultanenously transmitted within the same signal channel. These signals may be separated and detected at the receiver in two signals channels by means of synchronous demodulators without one corrupting the other. The degree of isolation between the two channels is dependent upon the deviation from the quadrature relationship between the two modulator signals. Any variation of phase and amplitude symmetry between the sidebands of each channel affects the phase for that channel and as a consequence the quadrature relationship between the channels. Deviation from the quadrature relationship reduces the isolation between the channels, thereby permitting crosstalk.
The addition of the double sideband suppressed-carrier quadrature channel to increase the information handling capacity in the TV channel, adapts well to conventional television transmission equipment. Further, the quadrature relationship, if properly maintained, reduces the sensitivity of standard receivers to the additional channel information.
Receivers presently in use employ Nyquist slope filters in conjunction with a picture signal demodulator to establish a flat response for the picture signals. In this system, the carrier signal is positioned at the -6dB point of the Nyquist slope filter, thus centering the quadrature channel at the -6dB point on the Nyquist slope of the receivers IF filter. To prevent distortion of the signals in the quadrature channel at detection, the signals in this channel must be distorted upon transmission in a manner that counteracts the Nyquist slope frequency response. In the prior art, this was accomplished by passing the additional information signals through a filter having a ramp frequency response with a slope that is the mirror image of the Nyquist slope filter with the signal frequencies centered about the -6dB point of one slope of the Nyquist filter. This procedure requires that the frequency response of the two mirror image slopes be very closely match, a condition very difficult to achieve.